1. Field of the Invention
The present invention relates in general to antennas, used for transmitting and receiving radio signals in the frequency range of microwave to millimeter-wave bands, having integral power lines and high frequency circuits for signal processing. The present invention relates, in particular, to a microstrip antenna of compact and high-flexbility design, comprising a plurality of radiation elements, provided in a common unit, for use in different frequency bands.
Such an antenna would find applications in personal communication systems including entry-control and security cards, terminals for ATM wireless access (AWA), remote-access terminal and others. Because of its compact and low-profile features, the antenna is also suitable for use in interior LAN, needing a multi-band system to overcome the operational problems of fading and shadowing caused by multi-pass interference being experienced in such a network.
2. Description of the Prior Art
Microstrip antennas are being used for radar, mobile and satellite communication systems, because of their compact, thin and light weight features.
Microstrip antennas are particularly suitable for use as active antennas. Active antenna is an antenna having all of the necessary components, such as an antenna element, a feeding circuits, active devices or active circuits, integrally provided on a monolithic substrate, thus producing a compact, low cost and multi-function antenna equipment.
FIG. 14A is a perspective view of an example of the configuration of the conventional microstrip antennas, and FIG. 14B is a cross sectional view through a plane at Axe2x80x94A in FIG. 14A. The device includes an radiation element 102, a ground plane 101 and a dielectric film member 100. The radiation element 102, the ground plane 101 and the dielectric film member 100 constitute a microstrip antenna 104. A strip conductor 103, together with the ground plane 101 and the dielectric film member 100 constitute a microstrip line 105. A signal propagated through the microstrip line 105 couples with the antenna element 102 in accordance with the electromagnetic field generated by the microstrip line 105 and feeds the microstrip antenna 104. The microstrip 104 is a type of resonators, and the generated radio waves are radiateted into the free space.
However, in this type of microstrip antenna, the microstrip line 105 locates in the direction of radiation of the waves with respect to the ground plane 101, leading to an undesirable problem that the unnecessary radiation from the microstrip line adversely affects the radiation field of the microstrip antenna.
To resolve this problem, there has been a suggestion for a different configuration of microstrip antenna such as the one presented in FIGS. 15A, 15B.
FIG. 15A is a perspective view of another example of the conventional microstrip antennas, and FIG. 15B shows a cross sectional view through a plane at Axe2x80x94A in FIG. 15A. The device includes an radiation element 112, a ground plane 111 and a first dielectric film member 110. The radiation element 112, the ground plane 111 and the first dielectric film member 110 constitute a microstrip antenna 114.
The device also has a strip conductor 113 and a second dielectric film member 115. The strip conductor 113, the second dielectric film member 115 and the ground plane 111 constitute a microstrip line 116. Also, a slot 117 is fabricated on the ground plane 111.
The signal propagated through the microstrip line 116 couples with the radiation element 112 in accordance with the electromagnetic field generated by the microstrip line 116 through the slot 117, and feeds the microstrip antenna 114.
The unnecessary radiation from the microstrip line 116 in the microstrip antenna shown in FIGS. 15A and 15B generates little adverse effects on the radiation field of the microstrip antenna 114, because the ground plane 111 intervenes and blocks the parasitic signals from the microstrip line 116 (acting as the power line) affecting the performance of the radiation element 112.
The properties of the microstrip antennas shown in FIGS. 14A, 14B and FIGS. 15A, 15B can be obtained from the dielectric constant, the dielectric dissipation factor (tanxcex4) and the thickness (h) of the first dielectric film substrate 110 (or 100), and the conductivity ("sgr") of the radiation element 102 or 112.
FIG. 16 is a graph showing the relationship between the radiation efficiency and no-load Q of a circular microstrip antenna without considering the effect of the surface wave (refer to K. Hirasawa and M. Haneishi, xe2x80x9cAnalysis, Design, and Measurement of Small and Low-Profile Antennasxe2x80x9d, Artech House, Norwood, Mass. 02062).
In this case, if S is the voltage standing wave ratio (VSWR), the bandwidth BW of the microstrip antenna is given by the following equation:
BW=(Sxe2x88x921)/Q0S0.5.
Therefore, the bandwidth of an antenna is inversely proportional to no-load Q, and, to obtain high performance properties in an antenna (high radiation efficiency, wide bandwidths etc.), FIG. 16 shows that it is preferable to have a thick dielectric film.
However, when the thickness h exceeds a certain value, the antenna performance becomes degraded because the loss caused by surface waves can no longer be ignored and higher order excitations are generated in the thickness direction. Therefore, in designing a high-performance antenna, the thickness of the dielectric film is the most important parameter. For microstrip antennas in general, the thickness of the dielectric film is chosen in the range of {fraction (1/50)}th to {fraction (1/20)}th of the free-space wavelength of the center frequency.
Although microstrip antennas are convenient and advantageous in many respects, they operate in a single frequency band and has a high Q-value, and therefore, if they are to find a wider application possibilities, increased bandwidth of the antenna is mandatory.
An attempt has been made to solve these problems by developing a dual-frequency microstrip antenna which is shown in FIGS. 17A, 17B.
FIG. 17A shows a plan view of a dual-frequency microstrip antenna, and FIG. 17B shows a cross sectional view through a plane at Axe2x80x94A in FIG. 17A. The antenna shown in this drawing is made by laminating two dielectric films 100 having one radiation element 102 between the two films and another radiation element 102 of the same size above the top film 100. Operating power is supplied to each of the radiation elements 102 through a power pin 200 formed on a ground plane 101 which is disposed opposite and away from the radiation element.
In this device, the boundary conditions of the electric and magnetic field components of the top and bottom dielectric films 100 coupled to each of the radiation elements 102 (disposed above and below the dielectric film member 100) are different, thereby providing different equivalent dielectric constants to enable the antenna to perform as a dual-frequency microstrip antenna.
More specifically, denoting the dielectric constant for the upper dielectric film 100 by xcex5r1 and that for the lower dielectric film 100 by xcex5r2, the equivalent dielectric constant of the upper radiation element can be approximated by (xcex5r1+1)/2 and that for the lower radiation element by (xcex5r1+xcex5r2)/2.
For popular film base material such as teflon or polytetrafluoroethylene (PTFE) whose relative dielectric constants xcex5r are about 2.55, there is little change in the equivalent dielectric constant, and the two radiation elements resonate in a close range of frequencies.
Therefore, there are cases in which an optimum film substrate thickness for one radiation element is not optimum for the other radiation element, resulting in degradation in the radiation efficiency and bandwidth of the overall antenna.
Another type of microstrip antenna designed to overcome the problems discussed above is shown in FIGS. 18A, 18B. FIG. 18A is a plan view of the antenna and FIG. 18B shows a cross sectional view through a plane Axe2x80x94A in FIG. 18A.
This type of antenna is provided with two different sizes of radiation elements 102a, 102b. Thus, the large radiation element 102a provided on the top film acts as a low frequency-band antenna, and the small radiation element 102b provided on the lower film acts as a high frequency-band antenna, thereby permitting to choose an optimum film thickness for each of the two dielectric films 100.
However, the dual-frequency microstrip antennas shown so far are designed with a power pin 200, thus leading to an undesirable effect of unnecessary radiation from the power pin affecting the radiation fields of the radiation elements.
A microstrip antenna shown in FIGS. 19A, 19B is an attempt to solve the problems posed by the antennas presented above. FIG. 19A is a plan view of another example of a dual-frequency antenna, and FIG. 19B is a cross sectional view through a plane at Axe2x80x94A in FIG. 19A.
The dual-frequency antenna is provided with a strip conductor 113 opposite to the top and bottom radiation elements 102a, 102b with an intervening ground plane 101. Each of the radiation elements 102a, 102b is coupled to the strip conductor 113 in its respective electromagnetic field, and are activated th rough a slot 117.
In this type of antenna also, two different sized radiation elements 102a, 102b are provided as in the case of the antenna shown in FIGS. 18A, 18B. Therefore, the top radiation element 102a acts as a low frequency antenna, and the bottom radiation element acts as a high frequency-band antenna, thereby permitting to choose an optimum film thickness for each of the two dielectric films 100.
However, in the conventional dual-frequency antennas presented so far, it has been difficult to fabricate more than three microstrip radiation elements having distinct operating frequency on a common dielectric film member to realize a multi-band antenna.
A microstrip antenna shown in FIGS. 20A, 20B is an attempt to solve these problems posed by the antennas presented above. FIG. 20A is a plan view of another example of a dual-frequency antenna, and FIG. 20B is a cross sectional view through a plane at Axe2x80x94A in FIG. 20A.
The antenna shown in FIG. 20A is provided with half-wavelength dipoles 201axcx9c201f having different resonant lengths printed on a dielectric film member 100, and opposite to the dipoles 201axcx9c201f, there is provided a strip conductor 113 with an intervening ground plane 101. Each of the dipoles 201axcx9c201f couples with the strip conductor 113 in its respective electromagnetic field through a slot 117, and is activated to generate signal waves of different frequencies, thereby performing as a multifrequency-band antenna.
However, because such a dual-frequency microstrip antenna is designed so that the half wavelength dipoles 201axcx9c201f are all printed on a common dielectric film member 100, and consequently, the frequency range for optimum performance is limited.
Further limitation of the antenna configuration based on the printed dipoles 201axcx9c201f is that, because the dipoles are meant to couple through the slot 117, there is a physical limitation to the number of dipoles which can be arranged within the coupling distance of the slot 117.
Therefore, it is difficult to fabricate simultaneously two microstrip radiation elements, having a widely separated operating frequencies, on a common dielectric film substrate using the approach shown in FIGS. 20A, 20B. In other words, the conventional dual-frequency microstrip antenna could not provide a multi-band capability in one device unit which performs well in any desired operating frequencies over a wide range of frequencies.
It is an object of the present invention to provide a compact and multi-function microstrip antenna of multi-band capability having a plurality of dielectric films of thicknesses, which are optimized for a plurality of different operating frequencies, fabricated on a monolithic substrate, so as to provide optimal performance in the operating frequency range of microwave to millimeter-wave, in such a manner as to facilitate the incorporation of integrated circuits within -the antenna.
The object is achieved in an antenna comprising: a multi-lamination microstrip antenna comprising: a plurality of dielectric laminations, each lamination comprising: a plurality of dielectric film members, each dielectric film member having a selected dielectric constant and a selected film thickness; an radiation element in contact with a film surface of one dielectric film member disposed in one lamination; and a ground film member, in functional association with the radiation element, in contact with another film surface of another dielectric film member disposed in another lamination different than the one lamination.
A further aspect of the invention is that the antenna may be provided with: a high frequency signal line, disposed opposite to the radiation element with the ground plane intervening between the high frequency signal line and the radiation element, in contact with a film surface of a dielectric film member in a lamination; and a slot formed on the ground film member for receiving input signals into the radiation element so as to enable the input signals to couple with the radiation element in accordance with the electromagnetic field generated by the high frequency signal line to drive the radiation element.
A further aspect of the invention is that the antenna may be provided with: a high frequency signal line, disposed opposite to the radiation element with the ground plane intervening between the high frequency signal line and the radiation element, in contact with a film surface of a dielectric film member in a lamination; a slot formed on the ground film member; and a conductor member for operatively connecting an end of the high frequency signal line with the radiation element, so as to produce a direct electrical connection to drive the radiation element.
An aspect of the antenna presented above is that the conductor member is an open ended cavity termed a via-hole.
Still another aspect of the antenna presented above is that the high frequency signal line is a microstrip line.
Still another aspect of the antenna presented above is that wherein the high frequency signal line is a tri-plate line.
Still another aspect of the antenna presented above is that the antenna is provided with a plurality of the radiation elements.
Still another aspect of the antenna presented above is that the antenna is provided with electronic circuit means for processing signals received by and transmitting from the radiation element, disposed in contact with a dielectric film member in a lamination.
Still another aspect of the antenna presented above is that the antenna is provided with electronic circuit means for processing antenna signals in operative contact with the ground film member.
Still another aspect of the antenna presented above is that a cavity space is formed by removing at least one dielectric film member.
Still another aspect of the antenna presented above is that circuit element means are provided at an interface between the dielectric film members.
Still another aspect of the antenna presented above is that the circuit element means are capacitors.
Still another aspect of the antenna presented above is that the circuit element means are resistors.
Still another aspect of the antenna presented above is that the dielectric film member is an alumina ceramic material.
The final aspect of the antenna presented above is that the dielectric film member is a polyamide film.
The outstanding features of the invention are summarized below.
The multi-lamination configuration of the present microstrip antenna allows radiation elements and ground planes to be placed between any dielectric film members, allows the film thickness to be selected freely and successively to meet a wide range of operating frequency bands, and allows equi-potential connection to the ground planes to be made through the corresponding open ended cavities or via-holes.
Therefore, it is possible to select a thickness of any dielectric film so as to optimize the performance of an radiation element for a specific operating frequency within a wideband frequency range, thereby producing a microstrip antenna capable of handling a large separation in the receiving and transmitting frequencies or in the operating frequency bands.
Lamination thickness and the film thickness can be freely selected to suit any requirement, therefore, it possible to produce a multifrequency-band antenna on a monolithic substrate, capable of operating in a plurality of different operating frequency bands within frequency range of microwave to millimeter-wave. Furthermore, the design of the antenna allows two or more radiation elements having slightly different resonant frequencies to be provided on different local regions of one thin dielectric film member, thereby permitting to increase the bandwidth of the antenna without degrading the performance of either radiation element.
Further, because the dielectric constant of each of the film members can be freely selected, selection of a film material having a high dielectric constant allows a reduction in the size of the radiation element, and the selection of a film material having a low dielectric constant material allows an increase in the gain and efficiency of the antenna.
By selecting such materials as alumina, aluminum nitride and silicon as the film material for their good thermal compatibility with the semiconductor substrate often used in such applications, it is not only possible to incorporate a microstrip antenna into a monolithic microwave integrated circuit (MMIC) formed on a semiconductor substrate, but also to produce a multi-function antenna by enabling such functions as active antenna to be incorporated into the antenna.
Further, such an active radiation element can be produced economically because capacitor and resistor elements can be placed at the interface between appropriate film members, thereby allowing to integrate active devices or active circuits with the antenna, without the need for installing chip condensers. This approach to circuit integration permits the use of Wilkinson divider to configure power combining circuits, thereby lowering the mutual coupling between antenna elements through the feeding circuits.
Further, the lamination structure allows a higher degree of freedom in designing power and biasing lines, compared with the conventional designs, because these elements can be placed in any suitable lamination and it allows more choices of circuits and means for power supply. More specifically, the lamination structure permits power to be supplied directly using the via-holes and signal coupling to be achieved through the slots. It offers a further advantage that power combining circuits can be either a microstrip or a tri-plate line configuration, so that shorting interconnections which are effective in suppressing undesired modes which plague the tri-plate line configuration can be achieved by using via-holes rather than soldering used in the conventional approach.